A demand exists for retroreflective materials i.e., materials capable of reflecting the predominant portion of light rays impinging thereon in a substantially parallel path toward the source of the light. A particularly efficient type of retroreflective element employs molded members of cube-corner formations for retroreflective structures.
Cube-corner reflectors molded from glass and more recently from acrylic resins have commonly been employed as safety devices on bicycles, automobiles and other vehicles.
Rowland in U.S. Pat. No. 3,689,346 describes a process in which retroreflective sheeting is produced on a continuous basis by applying transparent sheet material over a hardenable molding material previously deposited upon a moving mold surface, after which the molding material is solidified and bonded to the sheet material to produce a composite structure. The mold surface has an array of minute, contiguous cube-corner recesses therein, so that the sheeting correspondingly has a multiplicity of closely spaced cube-corner formations spaced about and projecting form a smooth surface of the sheet material, which provides the body portion thereof.
Currently, masters for the molds are flycut using machinery which must be accurate to approximately one millionth of an inch over the entire size of the master. Machines of this type can be used to make masters which are as large as 12".times.12" but not much larger. The larger the size of the machine, the greater the cost. A machine that can flycut a 12".times.12" master costs approximately $500,000 and a machine that will flycut a 18".times.18" master costs approximately $1,000,000. The cost increases exponentially with the machine size because of the accuracies required. Typically, electroformed replicas of the master are made and the replicas are assembled into larger molds using two steps.
The first step is to finish the edges of the 12".times.12" pieces using a very precise edge finishing process, such as lapping and polishing or fly cutting. Three 12".times.12" pieces are then assembled into a roughly 12".times.36" mold. The 12" long seams which are made are less than 0.001" in width and are acceptable for most applications and also the replica of the assembly is one continuous electroform. Strength at the seams is not a problem because the piece is all one electroformed piece of metal. The next step is to assemble the 12".times.36" molds to one another to form a belt. This step requires edge finishing equipment which must be extremely precise over the entire 36" width of the mold. It is possible to assemble larger molds at this point, for example, a 24".times.36" mold or a 36".times.36" mold, but the equipment required to edge finish and also the equipment needed to electroform larger parts, becomes more sophisticated and much more expensive.
Therefore it is typically preferred to weld the 12".times.36" molds together and, to achieve the strength required for the finished belt, the assembly is welded on both the back and the front of the seams. The light weld that is present on the front of the seams is approximately .015" to 0.025" wide, will not retroreflect and creates a seam that is more visible than desired in daylight and in retroreflected light. The textured surface of the seam causes scattering of the reflected light and makes the seam very visible, especially if the material made from the molds is metallized.